docs: Make left nav a scrollable fixed position element, so it scrolls independently from the content.
docs: Make left nav a scrollable fixed position element, so it scrolls independently from the content.

BUG=skia:4108

Review URL: https://codereview.chromium.org/1266943004

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot